In recent years, various new materials have been researched and developed. Whiskers are known as one of them. The term whisker refers to a single-crystal filament having a cross-sectional area of up to 7.9.times.10.sup.-5 square inches and a length of at least 10 times the average diameter. Since such whiskers are substantially free of transition in a crystal and are virtually flawless, and have tens to hundreds times the mechanical strength of polycrystals having the same make-up, the whiskers are very useful as reinforcing materials having properties such as high strength, outstanding rigidity and good abrasion resistance. So far potassium titanate whiskers, aluminum borate whiskers and the like have been put to practical use.
On the other hand, the existence of (priderite) titanate compounds is already known. For example, Japanese Examined Patent Publication No. 41,176/1987 discloses the preparation of powdery crystals of potassium aluminum titanate having the formula K.sub.2.0 Al.sub.2.0 Ti.sub.6 O.sub.16 --K.sub.2.4 Al.sub.2.4 Ti.sub.5.6 O.sub.16 by a process comprising grinding and mixing powders of potassium carbonate, aluminum oxide and titanium oxide, calcining the mixture at 1,200.degree. C. for 3 hours, grinding and mixing the calcined product and firing the powder at 1,200.degree. C. for about 20 hours. Japanese Examined Patent Publication No. 12,236/1983 discloses the preparation of prismatic single crystals of potassium lithium magnesium titanate 0.5 mm in maximum diameter and 10 mm long and having the formula EQU (K, Li).sub.2-y Mg.sub.1-y/2 Ti.sub.7+y/2 O.sub.16
wherein Y is a number which fulfills 0.ltoreq.Y.ltoreq.1 by a process comprising firing lithium carbonate, magnesium carbonate and titanium oxide and a powder mixture of potassium molybdate and molybdenum oxide as a flux at 1,300.degree. C. for about 4 hours and gradually cooling the mixture to about 900.degree. C. Further, H. Ohsato, J. Ceram. Soc. Jpn. 100 2! 148-151 (1992) contains a schematic diagram showing the condition of potassium aluminum titanate at ambient pressure, and discloses the relationship between a ratio of components constituting the potassium aluminum titanate and a lattice constant given by X-ray diffraction.
However, priderite titanate whiskers and whiskers having pores inside are unknown.
Generally, whiskers heretofore known as having high strength are about 0.01 to about 1.0 .mu.m in diameter and about 5 to about 30 .mu.m in length. Such size is closely correlated with the reduction in internal defects of whiskers and with the strength of whiskers. That is to say, the growth of crystals beyond said dimensional range tends to result in polycrystals and decreases the mechanical strength. Reversely the whiskers which fall short of said dimensional range produce insufficient effects as reinforcing materials. However, it is very difficult to control such microscopic shape.
Particularly in the preparation of potassium aluminum titanate, an aluminum oxide conventionally used as an aluminum source component is difficult to microscopically mix with the other components because of its high hardness. Further an aluminum oxide is a stable substance even at a high temperature and thus necessitates a prolonged reaction at a high temperature for the growth of crystals. Crystals grown by a prolonged reaction at a high temperature are unavoidably bulky. For this reason, it has been difficult to form whiskers having a microscopic shape.
A process using a powder mixture of potassium molybdate and molybdenum oxide as a fusing agent for the foregoing powdery raw materials was proposed but entailed difficulty in producing whiskers with a microscopic shape because of a high viscosity of the fusing agent at a high temperature which results in bulky crystals.